The present project involves the x-ray crystallographic structure determination of procaryotic heme proteins. The general objective is to extend our present understanding of the structure/function inter-relationships and evolution of these molecules. Structural investigation on members of two classes of proteins are currently underway. CYTOCHROME C555 OF CHL. THIOSULFATOPHILUM: This is an 85 residue c-type cytochrome which is derived from one of the most primitive known living organisms, an obligately anerobic green sulfur bacterium. It possesses both unique reactivity and physiochemical properties, its oxidoreduction potential being unusually low (plus 150mV) compared to other members of this class (plus 250 - 370mV). The structure of this protein has been determined at 2.7 Angstrom resolution and is currently being extended to higher resolution. CYTOCHROME C': These are dimeric bacterial heme proteins (2 x 14,000 mV) which posses physiochemical properties intermediate between those of c-type cytochromes and oxygen binding globins. The structure determination of the Rps. capsulata species (3.2 Angstrom) and the R. molishianum species (2.5 Angstrom) indicate that this molecule constitutes a new structural class of heme protein. The structural investigation of ligand-bound forms is underway, together with extension of the structures to higher resolution.